The thermo-regulating system of a human being aims to maintain a constant core temperature and skin temperatures within a range that varies between different body parts. Comfortable skin temperatures are within the range 28-33° C. Outside this temperature range, the body experiences discomfort.
The body controls the rate of heat exchange with the environment by regulation of the skin blood flow. Sweat production (evaporative heat loss) or shivering (heat production) sets in at larger deviations in body temperature.
The capacity and efficiency of the human thermo-regulating system is rather limited. Putting on or taking off clothes helps the body to stay within the comfortable temperature limits at different activity levels and ambient conditions for longer periods of time. However, it is not always appropriate or possible to put on or take off garments in a culturally acceptable way or it may be physically impossible or difficult. This applies particularly to garments like underwear or absorbent articles. Clothes and absorbent articles with built-in thermo-regulating properties would be able to maintain comfort without putting on or taking off clothes. Such clothing and absorbent articles would reduce discomfort caused by accumulation of sweat/moisture therein, and also shivering, which is rather unpleasant.
Integration of phase change materials (PCMs) in clothes is one way of achieving thermo-regulating properties. When skin temperatures increase, the PCM melts and absorbs heat released from the skin. Then, when the temperature drops, the PCM crystallizes and the stored heat is released again. In this way, variations in skin temperature can be suppressed and the temperature kept within the comfort zone. Not only products in the form of clothes and absorbent articles may benefit from incorporating PCM but also textiles used for e.g. for bed linen, pillow covers, blankets, furniture, car seats and footwear.
Textiles incorporating PCM may also be used in domestic and institutional applications like carpets and curtains in order to even out temperature fluctuations between day and night and thereby lower the energy costs for heating (night time) and air conditioning (day time).
The most common method of incorporating PCMs into textiles is by coating fabrics with a polymeric binder containing the PCM in microcapsules. The thermo-regulating effect is dictated by the coating weight. Further, the amount of microcapsules that can be added in the coating is limited, so the thermoregulating effect will be limited. In addition, applying microencapsulated PCM as part of a coating has several drawbacks besides the problem above and the high cost of microcapsules. Properties like air permeability and moisture permeability are impaired, which will affect the thermal comfort in a negative way. Further, increasing the add-on of coating results in a stiffer and less elastic fabric which is less comfortable to wear. Also surface properties like wetting may be negatively affected by the presence of a coating. This is especially important when dealing with training clothes or absorbent articles since a desired property of such articles is the ability of transporting bodily fluids on fibre surfaces.
The drawbacks associated with coatings can be avoided if the PCM microcapsules are incorporated inside the fibres. An added benefit is that the microcapsules are more durably bound to the fibres and can withstand laundering. Incorporation of microcapsules is possible in wet spun acrylic fibres and wet spun cellulose fibres but the thermal efficiency is rather low (less than some 10 to 30 J/g) since the amount of PCM that can be incorporated is restricted by factors such as spinability and sufficient fibre strength.
The dominating synthetic fibre used today is polyester, which is manufactured by means of melt spinning. Incorporation of microcapsules in standard meltspun fibres has so far been restricted for several reasons. The microcapsules must be able to withstand the high temperatures and shear forces encountered in the melt spinning process. Other reasons are the size of the capsules (1-10 μm) and the fact that particulate filler will increase the melt viscosity tremendously making melt spinning of thin fibres very difficult.
When one makes fibres with a content of PCM, it is the intention to obtain as high a thermo-regulating effect as possible per unit charge of PCM. In this perspective, the shell of the microcapsules is a ballast and an obstacle for energy transport. In order to achieve a fast exchange of energy between the skin of a human body and the PCM incorporated in a fibre, any unnecessary hindrance has to be minimized. Also, in order to load the fibrous material with as much PCM as possible, any unnecessary material component should be minimized.
If PCMs are to be used in melt spun fibres without being microencapsulated, that is, in a raw form, they have to be confined within the fibre. A solution is to use multi-component fibres with a core/sheath structure or a so called island-in-the-sea structure so that the PCM is trapped inside the fibres. However, a number of difficulties have to be overcome.
In “Effect of phase change material content on properties of heat-storage and thermo-regulated fibres nonwoven”, Indian Journal of Fibre & Textile Research, Vol 28, September 2003, pp. 265-269, a method of spinning fibres, comprising phase change material in raw form is described. Core/sheath fibres were melt spun with n-eicosane (as PCM) and a blend of polyethylene and ethylene-propylene copolymer in the core. The sheath was made from polypropylene. The maximum PCM content tested was 21 wt-% and a latent heat of 32 J/g of fibres was reached. However, only some 50-60% of the theoretically possible latent heat was realised indicating that a significant portion of PCM in the fibre core did not participate to the melting/crystallisation.
Further, WO 02/24992 A1 mentions that PCM in raw form is used when spinning fibres. But the examples show the phase change material enclosed in microcapsules and no examples with non-encapsulated phase change material are disclosed.
WO 2006/086031 A1 mentions the use of modified forms of ethylene-propylene co-polymers and polar copolymers (e.g. ethylene-co-vinyl lacetate polymer) to facilitate the dispersion of the phase change material in the core material. Fibres having a high content of phase change material and high values of latent heat are not disclosed.
U.S. Pat. No. 7,160,612 B2 also mentions PCM in raw form can be used when spinning fibres. The latent heat and the strength of the fibres are not satisfactory.
US 2007/0089276 A1 describes melt spun multi-component fibres incorporating PCM in raw form. The latent heat is not disclosed.
U.S. Pat. No. 7,241,497 A1 discloses a multi-component fibre comprising thermo-regulating material dispersed therein. The latent heat and the strength of the fibres are not satisfactory.
Polymeric phase change materials have also been used for spinning fibres, but although such phase change material has a higher viscosity than low molecular hydrocarbon waxes and thereby may not need to be mixed with a viscosity modifier, they are not very efficient as they possess quite low values of latent heat.
Thus, there is a need for fibres comprising high amounts of phase change material, where the fibres have a high latent heat combined with a good mechanical strength. Such fibres have not yet been described.
There is thus a need to develop multi-component fibres comprising phase change material with a good latent heat effect and having high strength. It is the aim of the present invention to solve the above problems.